15,665 research outputs found
Piezoconductivity of gated suspended graphene
We investigate the conductivity of graphene sheet deformed over a gate. The
effect of the deformation on the conductivity is twofold: The lattice
distortion can be represented as pseudovector potential in the Dirac equation
formalism, whereas the gate causes inhomogeneous density redistribution. We use
the elasticity theory to find the profile of the graphene sheet and then
evaluate the conductivity by means of the transfer matrix approach. We find
that the two effects provide functionally different contributions to the
conductivity. For small deformations and not too high residual stress the
correction due to the charge redistribution dominates and leads to the
enhancement of the conductivity. For stronger deformations, the effect of the
lattice distortion becomes more important and eventually leads to the
suppression of the conductivity. We consider homogeneous as well as local
deformation. We also suggest that the effect of the charge redistribution can
be best measured in a setup containing two gates, one fixing the overall charge
density and another one deforming graphene locally
Large Magnetic Moments of Arsenic-Doped Mn Clusters and their Relevance to Mn-Doped III-V Semiconductor Ferromagnetism
We report electronic and magnetic structure of arsenic-doped manganese
clusters from density-functional theory using generalized gradient
approximation for the exchange-correlation energy. We find that arsenic
stabilizes manganese clusters, though the ferromagnetic coupling between Mn
atoms are found only in MnAs and MnAs clusters with magnetic moments 9
and 17 , respectively. For all other sizes, 3, 5-10,
MnAs clusters show ferrimagnetic coupling. It is suggested that, if grown
during the low temperature MBE, the giant magnetic moments due to ferromagnetic
coupling in MnAs and MnAs clusters could play a role on the
ferromagnetism and on the variation observed in the Curie temperature of
Mn-doped III-V semiconductors.Comment: 4 Pages, 3 Figures[1 EPS and 2 JPG files], RevTeX
On a Petrov-type D homogeneous solution
We present a new two-parameter family of solutions of Einstein gravity with
negative cosmological constant in 2+1 dimensions. These solutions are obtained
by squashing the anti-de Sitter geometry along one direction and posses four
Killing vectors. Global properties as well as the four dimensional
generalization are discussed, followed by the investigation of the geodesic
motion. A simple global embedding of these spaces as the intersection of four
quadratic surfaces in a seven dimensional space is obtained. We argue also that
these geometries describe the boundary of a four dimensional nutty-bubble
solution and are relevant in the context of AdS/CFT correspondence.Comment: 20 pages, TeX fil
Investigations on T violation and CPT symmetry in the neutral kaon system -- a pedagogical approach --
During the recent years experiments with neutral kaons have yielded
remarkably sensitive results which are pertinent to such fundamental phenomena
as CPT invariance (protecting causality), time-reversal invariance violation,
coherence of wave functions, and entanglement of kaons in pair states. We
describe the phenomenological developments and the theoretical conclusions
drawn from the experimental material. An outlook to future experimentation is
indicated.Comment: 41 pages, 9 figures. See arXiv:hep-ph/0603075 for an enlarged versio
Force-extension relation of cross-linked anisotropic polymer networks
Cross-linked polymer networks with orientational order constitute a wide
class of soft materials and are relevant to biological systems (e.g., F-actin
bundles). We analytically study the nonlinear force-extension relation of an
array of parallel-aligned, strongly stretched semiflexible polymers with random
cross-links. In the strong stretching limit, the effect of the cross-links is
purely entropic, independent of the bending rigidity of the chains. Cross-links
enhance the differential stretching stiffness of the bundle. For hard
cross-links, the cross-link contribution to the force-extension relation scales
inversely proportional to the force. Its dependence on the cross-link density,
close to the gelation transition, is the same as that of the shear modulus. The
qualitative behavior is captured by a toy model of two chains with a single
cross-link in the middle.Comment: 7 pages, 4 figure
On Gravitational Radiation in Quadratic Gravity
We investigate the gravitational radiation emitted by an isolated system for
gravity theories with Lagrange density . As a formal result we
obtain leading order corrections to the quadrupole formula in General
Relativity. We make use of the analogy of theories with scalar--tensor
theories, which in contrast to General Relativity feature an additional scalar
degree of freedom. Unlike General Relativity, where the leading order
gravitational radiation is produced by quadrupole moments, the additional
degree of freedom predicts gravitational radiation of all multipoles, in
particular monopoles and dipoles, as this is the case for the most alternative
gravity theories known today. An application to a hypothetical binary pulsar
moving in a circular orbit yields the rough limit by constraining the dipole power to account at
most for 1% of the quadrupole power as predicted by General Relativity.Comment: 14 Pages, 1 Figur
Soliton Stability in Systems of Two Real Scalar Fields
In this paper we consider a class of systems of two coupled real scalar
fields in bidimensional spacetime, with the main motivation of studying
classical or linear stability of soliton solutions. Firstly, we present the
class of systems and comment on the topological profile of soliton solutions
one can find from the first-order equations that solve the equations of motion.
After doing that, we follow the standard approach to classical stability to
introduce the main steps one needs to obtain the spectra of Schr\"odinger
operators that appear in this class of systems. We consider a specific system,
from which we illustrate the general calculations and present some analytical
results. We also consider another system, more general, and we present another
investigation, that introduces new results and offers a comparison with the
former investigations.Comment: 16 pages, Revtex, 3 f igure
Nonlinear modes in the harmonic PT-symmetric potential
We study the families of nonlinear modes described by the nonlinear
Schr\"odinger equation with the PT-symmetric harmonic potential . The found nonlinear modes display a number of interesting features. In
particular, we have observed that the modes, bifurcating from the different
eigenstates of the underlying linear problem, can actually belong to the same
family of nonlinear modes. We also show that by proper adjustment of the
coefficient it is possible to enhance stability of small-amplitude and
strongly nonlinear modes comparing to the well-studied case of the real
harmonic potential.Comment: 7 pages, 2 figures; accepted to Phys. Rev.
Shock propagation and stability in causal dissipative hydrodynamics
We studied the shock propagation and its stability with the causal
dissipative hydrodynamics in 1+1 dimensional systems. We show that the presence
of the usual viscosity is not enough to stabilize the solution. This problem is
solved by introducing an additional viscosity which is related to the
coarse-graining scale of the theory.Comment: 14 pages, 16 figure
Structure-dependent ferroelectricity of niobium clusters (NbN, N=2-52)
The ground-state structures and ferroelectric properties of NbN (N=2-52) have
been investigated by a combination of density-functional theory (DFT) in the
generalized gradient approximation (GGA) and an unbiased global search with the
guided simulated annealing. It is found that the electric dipole moment (EDM)
exists in the most of NbN and varies considerably with their sizes. And the
larger NbN (N>=25) prefer the amorphous packing. Most importantly, our
numerical EDM values of NbN (N>=38) exhibit an extraordinary even-odd
oscillation, which is well consistent with the experimental observation,
showing a close relationship with the geometrical structures of NbN. Finally,
an inverse coordination number (ICN) function is proposed to account for the
structural relation of the EDM values, especially their even-odd oscillations
starting from Nb38.Comment: 11 pages and 4 figure
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